Colon cancer is the second leading cause of cancer mortality among adult Americans, accounting for 150,000 new cases and 70,000 deaths annually in the U.S. population. Each American carries a 7% lifetime risk of developing colon cancer. An essential feature of colon cancer, which carries the promise that colon cancer can be prevented in the population, is that colon cancer arises from a precursor, the adenomatous colon polyp. A pilot project is in progress to identify and to collect blood from 300 sib pairs in which both sibs are known to have in the present or to have developed in the past a colon neoplasm (either an adenomatous colon polyp or a colon cancer). After the DNA from the blood samples has been typed for marker loci spanning the human genome, the data will be analyzed to identify one or more major genes that predispose to the formation of colon polyps, and hence the existence of colon adenoma susceptibility alleles.